The present invention relates to a crystal puller for growing single crystal semiconductor material, and more particularly to a heat shield assembly incorporated in a crystal puller for growing such crystals at elevated pull rates.
Single crystal semiconductor material, which is the starting material for fabricating many electronic components, is commonly prepared using the Czochralski (xe2x80x9cCzxe2x80x9d) method. In this method, polycrystalline semiconductor source material such as polycrystalline silicon (xe2x80x9cpolysiliconxe2x80x9d) is melted in a crucible. Then a seed crystal is lowered into the molten material (often referred to as the melt) and slowly raised to grow a single crystal ingot. As the ingot is grown, an upper end cone is formed by decreasing the pull rate and/or the melt temperature, thereby enlarging the ingot diameter, until a target diameter is reached. Once the target diameter is reached, the cylindrical main body of the ingot is formed by controlling the pull rate and the melt temperature to compensate for the decreasing melt level. Near the end of the growth process but before the crucible becomes empty, the ingot diameter is reduced to form a lower end cone which is separated from the melt to produce a finished ingot of semiconductor material.
To increase throughput of the crystal puller, it is desirable to increase the pull rate xe2x80x9cvxe2x80x9d at which the crystal is pulled up from the melt. However, simply increasing the pull rate, by itself, can be detrimental to the growth and quality of the crystal. For example, an increase in pull rate can result in distortion of the ingot diameter if the ingot is not given sufficient time to cool and solidify as it is pulled up from the melt.
Also, some wafer quality characteristics, such as Gate Oxide Integrity, are effected by a change in pull rate. Silicon wafers sliced from the ingot and manufactured according to conventional processes often include a silicon oxide layer formed on the surface of the wafer. Electronic circuit devices such as MOS devices are fabricated on this silicon oxide layer. Defects in the surface of the wafer, caused by the agglomerations present in the growing crystal, lead to poor growth of the oxide layer. The quality of the oxide layer, often referred to as the oxide film dielectric breakdown strength, may be quantitatively measured by fabricating MOS devices on the oxide layer and testing the devices. The Gate Oxide Integrity (GOI) of the crystal is the percentage of operational devices on the oxide layer of the wafers processed from the crystal.
One way to improve GOI is to control the number of vacancies grown into the ingot upon solidification of the ingot as it is pulled up from the melt. It is understood that the type and initial concentration of vacancies and self-interstitials, which become fixed in the ingot as the ingot solidifies, are controlled by the ratio of the growth velocity (i.e., the pull rate v) to the local axial temperature gradient in the ingot at the time of solidification (Go). When the value of this ratio (v/Go) exceeds a critical value, the concentration of vacancies increases. Thus, to inhibit an increase in the concentration of vacancies, i.e., to avoid increasing the ratio v/Go, the axial temperature gradient at the solid-liquid interface must be correspondingly increased if the pull rate v is increased.
To this end, U.S. Pat. No. 5,316,742 discloses a single crystal puller apparatus having, as shown in FIG. 3 thereof, a first (outer) screen in the growth chamber arranged for surrounding the growing ingot as it is pulled up from the melt. The outer screen is constructed of silicon carbide coated graphite. A layer of insulating material constructed of carbon felt covers the inner surface of the outer screen. A second (inner) screen is arranged to surround the growing ingot intermediate the ingot and the outer screen in spaced relationship with the outer screen and insulating material. The inner screen is disclosed as being constructed of graphite (i.e., carbon). A cooling system comprising a pipe is wound around the inner screen for carrying cooling fluid therethrough to cool the inner screen. Accordingly, less heat is radiated by the screen toward the growing ingot, thereby increasing the axial temperature gradient of the ingot as it is pulled up from the melt. However, graphite has a high coefficient of radiation and, as such, a substantial amount of radiant heat from the growing ingot radiated to the inner screen is radiated back toward the ingot instead of being transferred to the cooling system. Thus, the crystal puller apparatus disclosed in this patent is not as efficient as desired.
Among the several objects and features of the present invention may be noted the provision of a heat shield assembly for a crystal puller which facilitates the growth of monocrystalline ingots from molten semiconductor source material at elevated pull rates; the provision of such a heat shield assembly which increases the axial temperature gradient of the monocrystalline ingot at the liquid-solid interface; and the provision of such a heat shield assembly which is shielded against molten silicon splashed from a source of molten silicon in the crystal puller and provides some protection against damage caused by inadvertent immersion of the heat shield assembly in the molten silicon.
In general, a heat shield assembly of the present invention for use in a crystal puller for growing a monocrystalline ingot from molten semiconductor source material comprises an outer reflector interposed between the ingot and the crucible as the ingot is pulled from the molten source material. A cooling shield is interposed between the ingot and the outer reflector as the ingot is pulled from the molten source material. The cooling shield is exposed to heat radiated from the ingot for increasing the rate at which the ingot is cooled, thereby increasing the axial temperature gradient of the ingot as the ingot is pulled from the molten source material. The outer reflector generally shields the cooling shield from heat radiated by the crucible.
Other objects and features of the present invention will be in part apparent and in part pointed out hereinafter.